A driving machine is connected to one end of a rotor of an electric rotating machine. To excite a core of the rotor, external power is supplied to a collector ring installed at the other end of the rotor. The power is supplied to a rotor coil through studs inserted into holes formed in a shaft in a radial direction and center hole conductors inserted into a center hole in the shaft. An increased output from the electric rotating machine also increases power supplied to the rotor coil, so that a current flowing through the rotor coil also increases. In order to improve thermal efficiency of facilities, a system has recently been beginning to be adopted in which driving machines are connected to opposite ends of the rotor of the electric rotating machine to transmit a driving torque to the rotor through the end of the rotor at which the collector ring is provided. The use of this system involves a high transmission torque applied to an end of a side of the rotor from which power is fed to a field circuit.
Also in a system in which no collector ring is provided and a brushless exciting apparatus is connected instead to the end of the rotor to enhance maintanability, an output from the brushless exciting apparatus increases with the electric rotating machine output increase and as a result transmission torque increases.
For the above-described increased current, each part of the current circuit can be settled by increasing sectional area. For example, for the studs, a sufficient sectional area can be provided according to the increased current by increasing the diameter of each of the studs or the number of the studs to increase the sectional area of the current circuit. However, the shaft needs to have shear strength sufficient to resist a transmission torque applied to the rotor, so the holes which extend from the center hole in the shaft to an outer periphery of the shaft for studs are inhibited from being formed in the shaft depending on the magnitude of the torque. In addition, the diameter of the shaft is inhibited from being increased to increase the shearing strength at the end of the rotor at which the collector ring with a peripheral speed limit is provided.
In a structure that eliminates the need for the radial holes for studs, conductors are arranged in the radial direction of the rotor and bolted and thus electrically connected to the center hole conductors.
However, in this structure, a contact area necessary and sufficient to accommodate the increased current fail to be provided simply by using the area of the end surfaces of the center hole conductors.